As the scaling of circuit patterns for semiconductor devices progresses, so does the scaling of mask patterns on photomasks. When the width of a mask pattern is reduced so that it is near the film thickness of the mask pattern, the focus position (best focus) at which the sensitivity to a focus shift is minimized becomes different depending on the shape of the mask pattern. Thus, it has become difficult to make the best focus uniform over the entire photomask, therefore, it has become difficult to reduce the dimension error of a resist pattern to fall within the allowable range over the entire pattern region of the photomask.